1. Field of the Invention
The present invention relates generally to FM communications equipment and more particularly to an improved squelch circuit for use with an FM demodulator to eliminate noise bursts at the beginning and end of a received transmission.
2. Description of the Prior Art
One of the most troublesome noise problems in radio communications is the noise bursts which frequently appear at the beginning and/or end of a demodulated transmission due to the finite times required to detect the commencement or termination of a signal and to thereafter activate or deactivate the receiver output circuits. These bursts are caused by short periods of noise signal, or settling transitions from noise-to-carrier-on and carrier-on-to-noise, which are allowed to pass through the audio circuit due to the inability of prior art receiver circuits to instantaneously recognize the presence or absence of carrier signal and to respectively open or close the audio circuit to prevent the passage of the noise components therethrough. Various types of squelch circuits have been used in the past in attempts to alleviate the problem but most such circuits merely attempt to mute the undesirable noise bursts rather than to eliminate them.